Just as there are marked differences in the structure and composition of hard PMMA, silicone, silicone copolymer gas-permeable contact lenses and soft contact lenses, there are also marked differences in the maintenance, care and treatment of the various types of hard, silicone, silicone copolymer and soft lenses. While patient care and treatment of hard contact or conventional contact lenses is relatively simple and uncomplicated, the proper care and treatment of silicone and silicone copolymer lenses (gas-permeable) and the newer soft and hydrophilic lenses has proved to be more complex, time consuming and costly to the patient.
The primary difference between the conventional hard contact lens and the silicone copolymer lenses and the more complex soft lenses is the hydrophobic nature of the silicone copolymer lenses and marked increase in the polar or water attracting centers of the hydrophilic gel material from which the soft contact lenses are made. It is this property of the hydrophilic gel lens that gives the soft lens its own unique physical properties and clinical behavior. This polar or water attracting center of the gel material is represented in the hydroxyethyl methacrylate bond as a hydroxyl group (--OH) which attracts and holds large amounts of water. It is this high water content held in the expanded matrix of the hydrophilic gel lens which leads to the special difficulties in cleaning and disinfecting or asepticising the soft hydrophilic lens. The hydrophilic nature of soft contact lenses makes the lenses vulnerable to bacterial contamination. While studies have demonstrated that bacteria cannot penetrate the actual intramolecular pores of the hydrophilic lens, except in defective lenses, the bacteria have an affinity for protein and tear deposits on the surfaces of the lens matrix. In particular, the tears and fluids absorbed by the soft lenses serve as excellent bacterial culture media. If defects or nicks occur in the lenses either during manufacture or subsequent patient wear, bacteria may find a haven to grow and be sheltered from superficial lens cleaning and disinfection.
Potentially harmful fungi are also a possible danger to the soft contact lens. Fungi, like bacteria, can thrive in tear secretions, other fluids or deposits and penetrate the lens material directly if enzymatic degradation of the lens material has taken place.
Similarly, any substantial residual proteinaceous or tear secretion deposits or lipid deposits remaining in or on the lens may readily overwhelm and inactivate the most effective germicidal components of a disinfecting system, and may thus serve to act as a growth media for a variety of potentially harmful microorganisms and fungi. Therefore, it is important that prior to storing the soft contact lenses in a disinfecting solution, protein and lipid deposits be removed from the lens surfaces so that the disinfectant properties of the sterilizing solution or method will not be overwhelmed by gross organic or inorganic deposits and pollutants. Therefore, an effective cleaning step or steps is an essential and mandatory part of any effective soft lens treatment and maintenance regimen.
Numerous methods of cleaning soft contact lenses to remove only protein deposits exist. For example, the soft lenses can be rinsed in tap water in an attempt to remove protein deposits. Tap water rinsing is virtually ineffective, removing only from about 1% to about 10% of the accumulated debris on the lens surfaces. Boiling the soft contact lenses in a saline solution is partially effective, removing generally from about 5% to 15% of the debris located on the lens surfaces. Other cleaning methods include the use of hydrogen peroxide and sodium chloride solutions. Hydrogen peroxide is ineffective since it will actually oxidize and change the color of the lenses. Use of high concentrations of sodium chloride aids in removing some encrustations because of the friction created in rubbing the lenses with a sodium chloride solution. However, this method is harmful to the lenses because scratching of the lens surfaces occurs. Enzymatic cleaners do not remove deposits such as salts, lipids and mucin. In addition, the enzymatic cleaners may also discolor the soft contact lenses. A more detailed discussion of various types of cleaning procedures and compositions is found in the April 1978 issue of Review of Optometry in a series of three articles, one each by: Irving J. Arons; Jerome S. Lieblein, O.D.; and Frederick D. Kleist and Jon C. Thorson, M.D.
Therefore, a need has arisen for an effective composition to remove protein, lipid and other deposits which tend to remain on gas permeable and soft contact lenses after a wearing period has been completed. A need has also arisen for a gas permeable and soft contact lens cleaner that can be easily rinsed from the lens surfaces after cleaning has been accomplished and that does not contain components which significantly bind to the lens or are otherwise deleterious to the lens material.
Since contact lenses are susceptible to bacteria and fungi, the cleaner should contain a preservative system to insure that the cleaning solution does not become contaminated by such organisms, the use of which could transfer the organisms to the lenses. Compounds such as thimerosal potassium sorbate, chlorhexidine, and sorbic acid when used as the main preservative, which are known for use in preservative systems for maintaining a solution sterile or essentially sterile have been used. However, these compounds have drawbacks in that they can discolor lenses and also be concentrated in the lens matrix and cause irritation, sensitization, excessive burning and red eye, which can prevent the patient from wearing the lenses. With the advent of extended wear lenses, it becomes even more important to avoid such problems, since those lenses can remain in the eye for several weeks.